1. Field of the Invention
The present invention is directed to a process for promoting the interaction between carbon black and rubber. More particularly, the present invention is directed to the use of azocarbonamide as a promoter of the interaction between carbon black and rubber.
2. Description of Related Art
Fillers, such as carbon black, are added to elastomeric compounds for a variety of reasons. They act as a low cost diluent and as reinforcing agents, giving higher-modulus, higher strength and greater wear resistance. The interaction between filler and an elastomer matrix is also very important to the enhancement of desirable properties such as hysteresis, and wear resistance. It is believed that when the interaction between the carbon black filler and the polymer matrix is increased, dynamic properties are improved. This is generally evidenced by lower hysteresis at elevated temperatures that would result in lower rolling resistance when the rubber is used to make automobile tires. Increased interaction between the polymer and filler can also result in cured rubber with lower heat build-up.
Interaction of rubber to the filler also results in changes in other properties. When rubber and carbon black interact, the amount of bound rubber increases. This is due to some of the polymer being strongly bonded to the surface of the carbon black. This is demonstrated by dissolving the uncured rubber in a good solvent, leaving the carbon black and bound polymer as a gel. In the absence of interaction, this quantity of gel will be minimal, and when interaction increases the amount of gel will increase. The increase in the amount of bound rubber gel in the uncured compound may be taken as evidence of increased interaction between the filler and polymer.
In the absence of interaction between filler and its elastomeric matrix, the filler forms a loosely bonded network within the matrix, which remains after curing. When the dynamic storage modulus, designated G′, is measured in the cured rubber sample, the filler network acts to increase the modulus at low strain. As the applied strain on the rubber sample is increased, the bonds that form this filler network are broken, and it no longer contributes to the modulus. Thus, in the presence of small or low interaction between filler and the elastomer matrix, the dynamic storage modulus, G′, will diminish as the applied strain is increased. This is known as the Payne Effect. As the filler to polymer matrix interaction increases, the filler-filler network should be decreased in the final cured elastomer. Thus when applied strain is increased as the dynamic measurement is made, the storage modulus, G′, does not decrease as rapidly with an increase in the strain. The diminution of the Payne Effect is also taken as evidence that increased filler-polymer interaction has taken place. Another way to measure this is by the % G′ Retained, which is simply the ratio of high strain/low strain, where higher is better.
Similarly, when the modulus of a cured sample is measured in simple extension, the modulus will increase as the strain increases. When a sample that has increased filler interaction to the matrix is compared to a control, the ratio of the modulus at high strain to the modulus at low strain will be higher. Thus, an increase in the ratio of the modulus at 300% extension to the modulus at 5% extension, (M300/M5), may be taken as evidence that additional interaction has taken place.
In the past some other chemicals have been added to rubber to improve the interaction of carbon black with the rubber matrix. N-Methyl-N,4-dinitrosoaniline was used commercially1, but was discontinued due to concerns about toxicity. (See, Leeper et al., Rubber World, 135:413–28, (1956); Walker et al., Rubber Age, 90:925–31(1962)). N,N′-bis (2-methyl-2-nitropropyl)-1,6-diaminohexane has been reported to be useful and is being sold as Sumifine 1162. (See, Yamaguchi et al., Rubber World, 199:30–38(1989)). Several other materials have been proposed as being useful, but so far have not attained any commercial sigificance. Benzofurazan oxide has been suggested, but it evolves a strong undesirable odor during processing. (See, D. F. Graves, Rubber Chem. TechnoL, 66:61–72, (1993); and U.S. Pat. Nos. 4,751,271; 4,975,497; and 5,001,171). Para-amino-benzenesulfonyl azide has also been suggested as a carbon black promoter, but so far it is not available commercially. (See, Gonzalez et al., Rubber Chem. Technol. 69:266–72 (1996). Benzimidazolinones, hydroxybenzimidazole oxides and related materials have also been reported to be effective. (See, U.S. Pat. Nos. 4,761,446 and 4,762,870).
Azodicarbonamide has been known to the rubber industry for some time. It is normally used as a chemical foaming agent for rubber and plastics, in order to obtain foamed rubber or plastic goods. In the manufacture of foamed rubber, azodicarbonamide is mixed with the uncured rubber polymer, along with curatives, antidegradants, and other auxiliary additives. The rubber is then heated to decompose the azodicarbonamide to gaseous products and initiate the curing reactions in such a way that the gases are trapped in the cured rubber as bubbles to provide a cured foamed rubber product.
Bridgestone Corporation has disclosed the use of blowing agents in tread compounds to make a tread with closed cells that gives better traction on ice and snow. The specific invention disclosed is a foamed tread with cells of a specified size and density. When azodicarbonamide was used in their examples, they added activators to bring the decomposition point down to the normal curing range.
U.S. Pat. No. 4,255,536 discloses a copolymer of a conjugated diene/monovinylarene modified by reaction with a dihydrocarbyl azodicarboxylate.
U.S. Pat. No. 5,109,902 discloses a pneumatic tire comprising a toroidal carcass, a belt composed of at least two rubberized cord layers arranged radially outside the carcass, and a tread surrounding the periphery of the belt. The tread rubber comprises a base rubber made of a foamed rubber, and a cap rubber made of a non-foam rubber. The cap rubber is arranged radially outside the base rubber.
U.S. Pat. Nos. 5,147,477 and 5,351,734 disclose a pneumatic tire which comprises a case and a tread which covers a crown portion of the case. The tread includes a foamed rubber which has a volume not less than 10% of that of the entire of the tread, and the foamed rubber is formed of a rubber composition which contains rubber components each having a glass transition point not higher than −60° C.
U.S. Pat. No. 5,571,350 discloses a pneumatic tire for all seasons comprising a tread provided with a foamed rubber containing a given amount of a particular resin such as crystalline syndiotactic 1,2-polybutadiene having specified hardness and average particle size, and having specified expansion ratio, average expanded cell size and storage modulus (E′) at −20° C. of a given range, and said to exhibit satisfactory braking and traction performances at not only dry-on-ice state but also wet-on-ice state while sufficiently holding the steering stability, durability and low fuel consumption in summer season.
U.S. Pat. No. 5,753,365 discloses a rubber composition including a rubber component and particulates of a crystalline syndiotactic-1,2-polybutadiene resin dispersed therein. The particulates have an average particle diameter of 1 to 500 μm, and the melting point of the crystalline syndiotactic-1,2-polybutadiene resin is not less than 110° C. The compounding ratio of the resin is 5 to 60 parts by weight relative to 100 parts by weight of the rubber component. The rubber composition is said to be useful for tires, other rubber articles, etc. With respect to tires in particular, a pneumatic tire includes a rubber composition used for a tread, wherein the rubber composition includes a rubber component and particulates of crystalline syndiotactic-1,2-polybutadiene resin. The matrix portion of the tread rubber other than the above particulates is preferably a foamed rubber.
U.S. Pat. No. 5,776,991 discloses a foamed rubber composition for pneumatic tires comprising at least one diene polymer as a rubber ingredient and includes closed cells in a matrix rubber, each of these cells being covered with a coat layer made from a given amount of a resin or resin composite having a JIS-C hardness of not less than 75, a particle size of 10–200 μm and a reacted conjugate diene unit content of not less than 10% by weight. In the production of the foamed rubber composition, the melting point or glass transition point of the resin or resin composite is restricted to at least 5° C. lower than the vulcanizing temperature.
U.S. Pat. No. 5,788,786 discloses a pneumatic tire in which a foamed rubber layer having closed cells is provided at a surface of a tire tread which substantially contacts at least a road surface, wherein the foamed rubber layer has closed cells whose average diameter is about 1 m to about 120 m and has an expansion ratio of about 1% to about 100%, a solid-phase rubber portion of the foamed rubber layer has a rubber composition in which at least both of a diene-type rubber and silica are mixed, and an amount of the silica is about 10 to about 80 parts by weight based on 100 parts by weight of the diene-type rubber. The pneumatic tire is said to exhibit excellent traction and braking abilities and controllability on snowy, icy and wet road surfaces.
U.S. Pat. No. 5,798,009 discloses a foamed rubber composition for tire comprising a particular amount of particles having specified hardness and average particle size and containing aluminum-bonded hydroxy group and/or silicon-bonded hydroxy group at its surface, and a particular amount of at least one specified silane coupling agent, and is said to develop excellent performances on ice. Further, pneumatic tires contain the foamed rubber composition as a tread rubber.